Musannaf Ibn Abi Shayba 1:1367

Isma'il ibn 'Ulayyah narrated to us, from Khalid, from Anas ibn Sirin, who said: A woman from the family of Anas experienced *istihada* (prolonged non-menstrual bleeding), so they ordered me, and I asked Ibn 'Abbas. He said: "As for what she sees of the 'Bahraini' blood (the dark, thick blood of menstruation), she should not pray. And when she sees the purity (the cessation of blood), even if for an hour of the day, she should perform *ghusl* (ritual bath) and pray."